1. Field of the Invention
The present invention relates to an optical recording medium comprising a substrate and a recording layer formed thereon comprising a specific azo metal chelate compound, and a recording and/or reproducing method using the above-mentioned optical recording medium.
2. Discussion of Background
The conventional write once read many recording medium employs as a recording material a cyanine dye (disclosed in Japanese Laid-Open Patent Applications 57-82093, 58-56892, 58-112790, 58-114989, 59-85791, 60-83236, 60-89842, and 61-25886); and a phthalocyanine dye (disclosed in Japanese Laid-Open Patent Applications 61-150243, 61-177287, 61-154888, 61-24609, 62-39286, 63-37991 and 63-39888). In addition, for the recording medium as the compact disk of a write once read many type, there have been proposed the combination of a metal reflection layer and a cyanine dye (disclosed in Japanese Laid-Open Patent Applications 1-159842, 2-42652, 2-13656 and 2-168446); the combination of a metal reflection layer and a phthalocyanine dye (disclosed in Japanese Laid-Open Patent Applications 1-176585, 3-215466, 4-113886, 4-226390, 5-1272, 5-171052, 5-116456, 5-96860 and 5-139044) and the combination of a metal reflection layer and an azo metal chelate dye (disclosed in Japanese Laid-Open Patent Applications 4-46186, 4-141489, 4-361088 and 5-279580).
The wavelength of a laser used for a currently employed optical disk system of a write once read many type, which may be referred to as WORM disk system is in the range of 770 nm to 790 nm, and a recording medium for this disk system is constructed in such a manner that recording and reproduction can be performed in the above-mentioned wavelength range.
Sooner or later, it will become inevitable that the capacity of such a recording medium has to be significantly increased in accordance with the amount of information to be handled. It will also become inevitable that the wavelength of the laser beam used for the recording and reproduction has to be shortened.
However, no recording medium has been developed that shows excellent light resistance and preservation stability and can be subjected to the recording and reproducing operation by the optical pick-up using a laser beam of 700 nm or less.
Currently employed CD-R (CD-Recordable) systems are also constructed in such a manner that recording and reproduction can be performed by a laser beam with a wavelength in the range of 770 nm to 790 nm.
As in the case of the above-mentioned WORM disk system, it will also become inevitable that the capacity of the recording medium for use in the CD-R system has to be significantly increased in accordance with the amount of information to be handled, and that the wavelength of the laser beam used for the recording and reproduction also has to be shortened.
In CDs and CD-ROMs which are currently employed, Al is coated on the concave and convex portions of the substrate therefor and the wavelength-dependence of the reflectivity of Al is so small that even if the wavelength of the laser beam used therefor is shortened in the future, reproduction of information recorded in such CDs and CD-ROMs is possible.
However, the recording layer of the currently employed CD-R, which uses therein a dye with a maximum absorption wavelength in the range of 680 nm to 750 nm, is designed so as to exhibit a maximum reflectance when a laser beam with a wavelength of 770 nm to 790 nm is applied thereto, with the optical characteristics and constants and the thickness and structure of the recording layer taken into consideration.
Therefore, when a laser beam with a wavelength of 700 nm or less is applied thereto, the reflectance obtained from the recording layer is extremely small, so that the currently employed CD-R cannot cope with the shortening of the wavelength of the employed laser beam in the future. As a result, it will be highly possible that the information recorded by the currently employed CD-R system cannot be reproduced by such a future system.